1. Field of the Invention
The present invention relates to a rechargeable lithium ion battery and more specifically, to a rechargeable lithium ion battery in which power density is increased.
2. Description of the Related Art
Research of various secondary batteries, which can be used for a long period of time and rechargeable as a power source of various electronic devices and electric apparatuses, has been made. Among others, active research and development have been made for a rechargeable lithium ion battery having characteristics such that higher energy density and higher power density can be realized in comparison with secondary batteries commercially available such as nickel-cadmium storage batteries and nickel-hydrogen storage batteries. The rechargeable lithium ion battery is commercialized as a power source of portable electric devices such as a cellular phone, a camcorder and a laptop personal computer.
In addition, interest in an electric vehicle and a hybrid vehicle has been increasing so as to solve the problems of environmental pollution of the earth and greenhouse effect of the earth. Application of the rechargeable lithium ion battery as a power source of such vehicles has been expected.
In applying the rechargeable lithium ion battery to vehicles and the like, the battery is used as a battery module in which a plurality of the batteries are connected in series to obtain high power density. The battery module also has advantages of easy control and high stability.
The rechargeable lithium ion battery has important characteristics such as energy density (i.e. energy per unit weight or energy per unit volume), power density (i.e. power per unit weight or power per unit volume) and cycle characteristic. Japanese Laid-Open Patent Publications H11-31498 (published in 1999), H11-297354 (published in 1999) and H11-329409 (published in 1999) disclose technologies for improving the characteristics.